To address the possible danger or damageo objects by vibration (or a series of shocks or impacts) during transport or storage, different systems are employed to detect whether vibrations have occurred that are outside specific limits. For example, ordnance subjected to shocks or vibration may be prematurely armed or vehicles/machinery subjected to high stress may lose structural integrity and become unsafe for use. MEMS (Micro-Electro-Mechanical Systems) based accelerometers are now commonly used as vibration sensors on many military and consumer applications including safety systems for automobiles. An alternative is an electret accelerometer which has lower power requirements.
A high-g accelerometer (>10 g maximum range) is typically used for fuse, safe and arm applications in the military. This class of accelerometers is used for shock, vibration, and inertial measurements. MEMS-based accelerometers generally use capacitive sensing to detect the deflection of an inertial mass. The example shown in FIG. 10 is the Analog Devices ADXL250, which has been in production by Analog Devices of Norwood, Mass., since 1993. FIG. 10 is a perspective view, including enlarged inserts, of a conventional capacitive accelerometer. FIG. 10 shows a beam 1002, a spring 1004, capacitive sense plates 1006 and stationary polysilicon fingers 1008. Also shown in blown-up inserts in FIG. 10 are fingers 1010 and spring attachment 1012.
The ADXL250 employs two sensors that are on orthogonally oriented axes. Each one is a differential capacitor sensor that has a fixed plate of polysilicon fingers and an inertial mass consisting of a moving plate that responds to the acceleration. MEMS accelerometers are mounted in hermetically sealed packages that protect them from humidity. Sensors manufactured by Analog Devices, Motorola Semiconductor Products, Kistler Instrument Corporation, and Dallas Semiconductor have been subjected to a range of reliability tests by NASA (R. Ghaffarian, D. G. Sutton, Paul Chaffee, N. Marquez, A. K. Sharma and A. Teverovsky, “Thermal and Mechanical Reliability of Five COTS MEMS Accelerometers”), the results of said tests are hereby incorporated by reference herein in their respective entireties. The ADXL250 showed only minor parametric changes during thermal cycling from −65° C. to 185° C. and 2000 g shock testing for 30,000 shocks. Other sensors remained within nominal specifications during thermal cycling from −40° C. to 85° C.
As evident from the foregoing discussions, the accelerometer detection logic circuitry and software technology are mature and known in the art. However, in view of the diverse operating environments of such systems, they would need to meet stringent requirements in order to be useful. Some of the key requirements are: 1) vibration (shock, impact)/amplitude detection and recordation, with limits that can be customized for specific objects, 2) reliability under wide variations in temperature and humidity, and 3) powering the recordation system throughout the lifetime of the object (several years or more if needed). While the first two of these requirements may be addressed with mature technology, the third requirement is generally addressed by employing batteries to power the detection/recordation systems. However, powering the VPIR continuously would drain the batteries in a relatively short time thereby limiting its prospects for long term use. Thus, while a battery-powered impact recorder employing any of the aforementioned MEMS-based accelerometers is feasible, the inventors have found that the need to provide sustainable power for such systems is yet to be satisfactorily met.